Nex are great for low light and IQ, but have you held one with a decent size lens? Imo mirrorless is not that much smaller than a d5100. sure if you had an nex6 with a fast prime lens it would be smaller than a d5100 with prime lens, but still not pocketable imo. hence why i think they are too much of an ok at most things but not great at anything.

Plus a DSLR you will have significantly more native lens options.

I dont think there really is such a thing as a super zoom pocket camera that performs well in low light. The rx100 is as close as you can get but still lacks the super side of zoom.